1. Field of the Invention
The present invention relates to an optical fiber for Raman amplification that amplifies a signal light with a pumping light, an optical fiber coil formed by winding the optical fiber for Raman amplification around a bobbin, a Raman amplifier including the optical fiber for Raman amplification, and an optical communication system including the Raman amplifier.
2. Description of the Related Art
Recently, broadband Internet services are steadily spread, from which it is clear that-there is a need for a backbone network having even larger capacity. For the past 10 years, a wavelength division multiplexing (WDM) has become a mainstream technology for increasing a capacity of an optical-fiber communication system. Signal wavelength bands need to be enlarged to expand the limit of the capacity.
From among many elements that form an optical communication system, an optical amplifier is a factor that limits a signal wavelength band. Presently, an optical amplifier used in current optical communication system is an erbium-doped fiber amplifier (EDFA). The gain bandwidth is limited to a wavelength range of approximately 1,530 nm to 1,565 nm, which is called C band, and a wavelength range of approximately 1,570 nm to 1,610 nm, which is called L band. Therefore, application to a band that cannot be amplified by the EDFA is being considered. Meanwhile, a fiber Raman-amplifier using stimulated Raman scattering in an optical fiber is able to amplify an arbitrary wavelength by appropriately selecting a pumping wavelength. Currently, the Raman amplifier is not widely applied to the C band or the L band because conventional Raman amplifier is poor in amplification efficiency compared to the EDFA.
In the Raman amplifier, optical fiber is used as an optical amplifying medium, and signal lights are Raman amplified by supplying-a pumping light to the optical fiber. Specifically, the Raman amplifier employs stimulated Raman scattering that is a nonlinear optical phenomenon in optical fiber.
Therefore, a highly nonlinear optical fiber is preferably used as the optical fiber for Raman amplification. For example, a Raman amplifier disclosed in Japanese Patent Application Laid Open No. 2002-277911 employs a highly nonlinear optical fiber as the optical fiber for Raman amplification to perform lumped optical amplification. An absolute value of chromatic dispersion at a wavelength of a signal light propagated through the optical fiber for Raman amplification is in a range between 6 ps/km/nm and 20 ps/km/nm.
In designing the conventional Raman amplifier, in addition to gain and noise figure (NF) that are basic parameters of an optical amplifier, multi-path interference (MPI) noise caused by double Rayleigh back scattering (DRBS) and nonlinear phase shift (NLPS) have been particularly attracting an attention. The gain and the NF are basic specifications determined by the system design. Therefore, the object in designing a single amplifier is to reduce the MPI noise and the NLPS as much as possible, with a condition that the characteristics of the gain and the NF are constant. A pumping power required for the amplification is also a guideline in the design of the amplifier, which can be referred to as amplification efficiency.
Generally, as the amplification fiber becomes shorter, the MPI noise and the NLPS become smaller. However, at the same time, amplification efficiency degrades, making a trade off between these factors. To reduce the NLPS, a nonlinear coefficient (n2/Aeff,S) should be decreased, and to increase amplification efficiency, a Raman gain efficiency (gR/Aeff,R) should be increased, where n2 is a nonlinear refractive index, Aeff,s is an effective area for the signal light, gR is a Raman gain coefficient, and Aeff,R is an arithmetic average of effective areas at a signal wavelength and a pumping wavelength. As described above, the effective area is related to both the nonlinear coefficient and the Raman gain efficiency. Therefore, if either one of the nonlinear coefficient and the Raman gain efficiency is increased, the other increases as well, and therefore, there is a trade off between these characteristics as well. Under these conditions, the amplification fiber needs to be designed so that the characteristics, between which a trade off exists, are within tolerance levels.
Generally, nonlinear effects in optical fibers degrade transmission quality of WDM signals in many cases. Specifically, four wave mixing (FWM), self phase modulation (SPM), cross phase modulation (XPM), and stimulated Brillouin scattering (SBS) can be factors to degrade the transmission quality. In the FWM, a noise light that is similar to amplified spontaneous emission (ASE) is input to a detector with the signal light, thereby generating an intensity noise due to a random interference. The SPM and the XPM are phenomena in which a phase shift is generated according to a pattern of an intensity modulation signal because the refractive index of the glass is dependent on light intensity. Due to a combination of a dispersion of a transmission line and a temporal change of the phase shift (a change in instantaneous frequency), a waveform of the intensity modulation signal is distorted, resulting in an increased bit error rate.
A nonlinear phase shift (NLPS), which is generally used as a parameter for estimating a magnitude of the nonlinear effect, represents a magnitude of the SPM, and with only the NLPS, it is insufficient for estimating the effects of FWM and XPM that are largely affected by a dispersion of the optical fiber. For example, the FWM efficiency is low as long as the phase matching is not achieved. When using a highly nonlinear optical fiber in which an absolute value of chromatic dispersion at a signal light wavelength is in a range between 6 ps/nm/km and 20 ps/nm/km, as an optical fiber for Raman amplification, because the absolute value of the chromatic dispersion is not near zero, even if the NLPS is a relatively large value, it is possible to suppress the degradation of transmission performance of signal lights due to the FWM. Meanwhile, the magnitude of chromatic dispersion necessary for suppressing the XPM is assumed to be somewhat large compared to the case of the FWM, although the value is not confirmed.